Method of making photo engraving plate



United States Patent METHOD OF MAKING PHOTO ENGRAVING PLATE Robert T.Hendrich and Dudley L. OBrien, Jr., Cleveland, Ohio, assignors to TheWire Coating & Manufacturing Company, Cleveland, Ohio, a corporation ofOhio No Drawing. Application April 29, 1954, Serial No. 426,566

11 Claims. (Cl. 4143) From the very beginning of the acid etching typeof engraving, the engraver has been plagued with the lateral action ofthe etchant. This action is referred to as undercutting. Many schemesand devices have been suggested as remedies for this problem, but untilrecently, one method alone has maintained its position of practicalimportance. The method referred to is called powdering or powder bankingand is in essence the heart of the en graving process.

In practice the engraver etches the prepared engraving plate to amaximum depth as defined by the finest or most delicate details in thedrawing or photograph. If this maximum depth is exceeded, excessiveunder-cutting takes place rendering the plate useless for stereotypingoperations. Beyond this point the supporting structure of the charactermay be dissolved or so weakened that direct printing will cause it toseparate completely from the base metal. The engraver must not,therefore, exceed this maximum depth. He must stop the etching processand powder bank the engraving in order to protect the finest details.This is done by dusting a finely powdered thermoplastic resin over thesurface of the engraving and with a brush sweep the loose powder fromtop to bottom. This action banks the powder against the top edge of therelieved character. The engraving plate must then be heated to thefusing point of the resin and then cooled. The cooling is generally donewith an air blower and then with cold water. The plate must then bedryed and powder-banked again from left to right. This protects the leftside of the relieved character. The same process is repeated from rightto left and from bottom to top. At this point the engraver is ready toetch the plate deeper. The second etch, or bite as it is referred to hasits maximum depth limits, though somewhat broader, as did the precedingetch, or first bite. If a third bite is to be taken, the powder bankingoperation must be again performed. The number of bites to be taken andconsequently the number of powder-banks made is determined by the natureof the characters and the type of work for which the engraving isintended. Fine screen characters require relatively shallow engravingdepths, heavy line engravings which are to be used for stereotyping mustbe deeply etched. Engravings intended for direct printing can have lessdepth than those intended for stereotyping. Direct printing on finepaper stock requires less etching depth than direct printing on coarsestock, etc. In essence, a great deal of skill and time is required toproduce a commercial engraving by the standard methods.

Recently, a group of patents issued to Messrs. Easley and Swaze of which2,640,766 is representative and which seek to solve this problem ofundercutting by the inclusion in a nitric acid etching bath of acarboxylic acid or the esters or salts thereof. The functions of theaddition agent is not fully explained by the patentees, however, theprocess of such patents is accompanied by many disadvantages and highcost of operation because in order to utilize it to any extentwhatsoever, it is necessary to employ expensive machinery. In theutilization of the Easley-Swaze process, the plate to be etched issupported with its active face downwardly in an appropriate machinewhich contains the etching bath and which propels the etching medium athigh velocity against the downwardly directed face of the plate. Anysuch (ill for the production of a uniform product is complicated "iceand difiicult to maintain. Moreover, the baths provided in saidEasley-Swaze patents have very short shelf life since they degenerate inseven or eight hours whether used or not and furthermore a special hottop or burned in enamel is required. A further disadvantage to theEasley-Swaze process is that printing plates formed entirely ofmagnesium must be employed in order that the process may be successful.

It is a principal object of our invention to provide a process whicheliminates undercutting and which is not only a full solution to theproblem, but may be performed and carried out with a minimum of effortand at a minimum of cost.

It is a further and more particular object of the invention to provide abath of the character described which is stable over long periods oftime, relatively inexpensive, is not subject to becoming unbalanced inuse and may be utilized on a variety of printing plates.

Other objects of the invention will appear as the description proceeds.

To the accomplishment of the foregoing and related ends, said invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims, the following descriptionsetting forth in detail certain illustrative embodiments of theinvention, these being indicative, however, of but a few of the variousways in which the principal of the invention may be employed.

Broadly stated, the method of our invention may be described as themethod of etching the surface of an acid-soluble metal object havinglocal areas thereon masked with an acid-resistant coating whichcomprises supporting the object, with said surface thereon facing eitherupwardly or downwardly in an acidic bath which will corrode the unmaskedportion of said surface and which bath contains a finely-divided solidwhich is in soluble in the bath, but which will for one reason oranother, adhere to the work being corroded and actually build up at thejunction between the masked and unmasked portions to prevent corrosiveeffect of the etching bath in those areas and accordingly preventundercutting.

THE WORK OBJECT The invention will be described in conjunction with theformation of printing plates, although it will be quite evident to thoseskilled in the art that our invention has a wider field of utility.

The present invention is applicable to a wide variety of work materials,that is, the metals or alloys from which the sheet or other object to beetched is formed. The present method finds application to printingplates made entirely of magnesium. In its most refined and commerciallyacceptable form, our method is preferably utilized in the preparation ofprinting plates from a composite sheet in which the body is magnesium ormagnesium alloy and which has applied thereover a thin continuous layerof zinc. The subject matter of such a plate is described and claimed inour co-pending application, Serial No. 256,162, filed November 13, 1951,and therein are given processes by which such a plate may besatisfactorily produced. It is believed to sufiice for the presentdescription, to state that when a composite plate is used the base ormajor portion of the body is preferably magnesium or a magnesium alloyand the superficial coating of a thickness of not more than about .0008is deposited thereon so as to be smooth, uniform and adherent.

When using a magnesium base alloy for the body of the sheet, generallybest results will be secured if such alloy contains not more than about5% of aluminum.

The plate whether it be of a single composition such as made entirely ofmagnesium or magnesium alloy or whether it be the composite type justdescribed, will be prepared in the usual manner conventional in thepreparation of plates of this kind. Such preparation will include theforming of a design on the plate whether by scratching or otherwiseremoving an acid-resistant barrier in local areas or the application ofsuch a barrier in local areas only.

In any event this stage of the preparation of the plate is conventionaland forms no part of the present invention and, therefore, is believedneed not be described in greater detail.

THE ETCHlNG BATH The etching bath is preferably one which during itsattack on the plate will form a precipitate which is insoluble in thebath and capable of being inductively charged. We shall presently giveexamples of baths which although quite different in compositionnevertheless are both capable of forming such a precipitate.

While we thus prefer a bath which will form the precipitate in situ itis nevertheless within the contemplation of our invention to add to thebath a finely divided material which has the same properties describedabove requisite of the precipitate, namely that it must be an extremelyfinely divided form, it must be insoluble in the bath and preferablymust be capable of being inductively charged. There are many naturalgums and synthetic resins which adequately satisfy all of these needsand which, accordingly can be used for this purpose.

When pre-formed solid material in finely divided form is added to thebath, it will, of course be necessary to agitate mildly during theetching operation, inasmuch as such mild agitation is usually desirablefor the purpose of insuring uniformity of effect of the etching acid andalso to speed up the etching time or rate, the conditions which areotherwise maintained in the bath in carrying out our invention utilizingan added solid material are conventional to the processes currently inuse.

As indicated above, however, the preferred embodiment of our inventionutilizes a bath which will in situ form a precipitate as the etchingoperation proceeds.

Example 1 Aluminum nitrate (crystal) 11 oz. Ammonium persulfate 2 lbs.,5 oz. Water, sufficient to make 1 gal.

Various concentrations and ratios of aluminum nitrate to ammoniumpersulfate have been tried. As the ratio of nitrate to persulfateincreases, the etching rate decreases. As the ratio decreases, i. e.,the relative concentration of persulfate increases, the etching rateincreases as does the rate of formation of the protective bankingprecipitate. The best weight ratio of aluminum nitrate to ammoniumpersulfate is about 1 to 3. Concentrations are workable from 5% totalsalts to saturation, but the more dilute solutions are slower working.The etchant has been used at temperatures ranging from 40 to 200 degreesFahrenheit, but the most suitable range appears to be between 70 and 120degrees Fahrenheit.

The following is another example of a bath which may be usedsuccessfully in carrying out our method:

Example 2 KMnO4 .38 oz. HClO4 (70%) 4.00 fluid oz. Water To make 1 gal.

The baths referred to above which are each capable, when etching amagnesium base plate of forming a precipitate of the character referredto above may be used, as indicated, on sheets composed entirely ofmagnesium and magnesium alloys. When such a sheet is used, it ispreferable to support the sheet in the bath with its surface to beetched facing upwardly so that as the precipitate is formed during theetching operation, it will settle onto the work face. Because thechemical action decreases in intensity toward the areas where thesurface of the plate has been masked, the precipitate as it settles hasbeen found to have a tendency to collect at the point of juncturebetween the area to be etched and the area which has been masked andthis piling up continues as the etching depth progresses so that thereis, in effect formed in situ. a powder-bank like that referred topreviously as being produced manually.

As indicated above, our method finds its greatest usefulness in thepreparation of printing plates made from a magnesium base sheet to whichhas been applied a thin uniform adherent layer of zinc. When such aplate is used, it is best to first subject the masked or otherwiseproperly prepared plate to a short etch of a conventional nitric acidbath which will be sufficient to remove substantiallv all of the zinc inthe unmasked areas of the plate. The plate is then rinsed off and placedin a tray or tank of the type of bath referred to above which may beused in practicing our invention. When the base plate is thus formed ofmagnesium, and the plate has a coating thereon of zinc, there isprovided a two-component system establishing a primary cell having apotential of about 1.58 volts. As the precipitate which may beinductively charged is formed, it is electrostatically attracted to thejuncture line or the area of these primary cells and caused to bank inthe same manner that powder banking has been accomplished manually.

By having reference to the bath of Example 1, it will be noted that theprimary function of the ammonium persulfate is that of an oxidizingagent, and the precipitate formed in this bath is aluminum hydroxide.

In the case of zinc on magnesium, oxidation substances other thanammonium persulfate may be used, as for example the sodium, potassium,or aluminum persulfates, the peroxides, the perborates, permanganates,etc. The protective precipitates may be derived from manganese,aluminum, uranium, beryllium, or metals in the electromotive seriessituated between zinc and magnesium. Also included are complexes,compounds both organic and ingrganic which have oxidation potentials inthis interva As the surface coating becomes more noble, as for examplecadmium on magnesium, copper or cadmium on magnesium, etc., the possiblenumber of oxidizing agents and protective precipitates increases. As thebase metal becomes more noble the number of possibilities decreases, asdoes the etching rate.

It should be noted that whereas when operating on a plate composedentirely of magnesium so that the primary cell action is not present, itis practically necessary under all circumstances to support the printingplate with the surface to be etched facing upwardly in the printing bathso that the precipitate as it forms may settle on the plate. When,however, using a composite plate such as a magnesium plate coated withzinc, it is possible to carry on the self-banking etching method of ourinvention by having the face of the plate extending downwardly since thegalvanic action will hold the self-banking precipitate on the plate.

The etching rate increases rapidly with increasing temperature, but theprotective bank has a tendency to dislodge itself at the highertemperature levels.

The etching bath has been used successfully in standard equipment, andthe ordinary cold-top enamels are suitable resists for the etchant. Theetching chemicals themselves are quite stable, and have stood forseveral days without any apparent breakdown. By the protective bankingaction of the etchant any etching depths are possible, thus permittingthe engraver to handle all types of work from fine screen half-tones toline drawings, or combinations of both.

The etching baths of our invention may be used in conventional typeetching machines and it is found that at the end of 20 to 40 minutesdepending upon the temperature of the bath, the plate will have beenetched to a depth suitable for direct printing or stereotyping, as forexample, from about 20 to 30 thousandths of an inch. The etching rate isconsiderably increased when vigorous agitation is employed as in thecase of a standard splash-type etching machine. More control is possibleunder these conditions also, for premature fill-in of the half-toneareas can be prevented.

The principles of our invention are applicable to the etching of a widevariety of materials in addition to those referred to above merely asillustrative examples. It is thus within the contemplation of ourinvention to apply our meth ds to objects made of all types and kinds ofacid-soluble metals and alloys such as iron, stel, copper, tin,magnesium, zinc and the like as well as non-metallic objects such asthose formed of glass, enamel and the like.

Other modes of applying the principle of the invention may be employed,change being made as regards the details described, provided thefeatures stated in any of the following claims or the equivalent of suchbe employed.

We, therefore, particularly point out and distinctly claim as ourinvention:

1. The method of etching the surface of a metal object having localareas thereon masked with an acid-resistant coating and simultaneouslybanking the sides of the re lieved portions of the etched surface toprotect them against undercutting which comprises contacting suchpartially masked surface with an acidic aqueous medium which willcorrode the unmasked areas of said surface and thereby create anelectrical potential across the interfaces between the masked andunmasked portions of said surface, providing in said solution a hydrosoland continuing said contact of acidic medium with said surface in thepresence of said hydrosol to form a precipitate at said interfacescreating a bank to protect said relieved portions from undercutting.

2. The method of etching a printing image in a metal printing platehaving local areas thereon masked with an acid-resistant coating andsimultaneously banking the sides of the relieved portions of the etchedsurface to protect them against undercutting which comprises contactingsaid partially masked printing plate with an acidic aqueous medium whichwill corrode the unmasked areas of the plate and thereby create anelectrical potential across the interfaces between the masked andunmasked portions of said plate, providing in said solution a hydrosolwhich is insoluble in said acidic solution, and continuing said contactof acidic medium with said plate in the presence of said hydrosol toform a precipitate at said interfaces creating a bank to protect saidrelieved portions from undercutting.

3. The method of etching a printing image in a bimetallic printing platecomprising a base metal sheet and a metal surface coating having localareas thereon masked with an acid-resistant coating and simultaneouslybanking the sides of the relieved portions of the etched plate toprotect them against undercutting which comprises contacting saidpartially masked printing plate with an acidic aqueous medium which willcorrode the unmasked areas of the plate and thereby create an electricalpotential across the interfaces between the masked unetched surfacecoating metal areas of the plate and the unmasked etched base metalareas, providing in said solution a hydrosol which is capable of beinginductively charged with a polarity which will cause the hydrosol to bepreferentially attracted to said electrically charged interface areas ofthe plate, and continuing said contact of acidic medium with said platein the presence of said hydrosol to form a precipitate at saidinterfaces creating a bank to protect relieved portions of the plateformed by the etching process from undercutting.

4. The method of forming a printing plate which comprises providing ametal plate having a surface thereof coated with a metal which is lowerin the electromotive force series of metals than the metal of the basesheet, forming an image thereon of local areas masked with anacid-resistant coating, contacting said partially masked plate with anacidic aqueous medium which will corrode the unmasked areas of the plateand thereby create an electrical potential across the interfaces betweenthe masked and unmasked portions of said plate, providing in saidsolution a hydrosol which is insoluble in said solution, and continuingsaid contact of acidic medium with said plate in the presence of saidhydrosol to form a precipitate of the hydrosol at said interfacescreating a bank to protect relieved portions of the plate fromundercutting simultaneously with their formation.

5. The method of claim 4 wherein said hydrosol is provided in saidsolution by adding a preformed hydrosol tlllereto prior to said contactof the solution with the p ate.

6. The method of claim 4 wherein said hydrosol is provided in saidsolution by dissolving in the solution prior to contact of the solutionwith the plate an inorganic salt the cation of which is a metal having aposition on the electromotive force series of metals between said metalof the base sheet and the metal forming said coating.

7. The method of claim 2 wherein said plate is a monometallic plate.

8. The method of claim 2 wherein said plate is a bimetallic plate.

9. The method of claim 4 wherein said plate is supported with the maskedsurface thereof facing upwardly during said contact of the plate withthe acidic solution.

10. The method of forming an etched printing plate without need tomanually bank the plate which comprises providing a bi-metallic printingplate consisting of a base metal sheet and a metallic coating thereon ofa metal which has a position in the electromotive force series of metalsbelow the metal of the base sheet, providing an acidic solution whichwill corrode said base metal of said plate containing the inorganic saltof a metal which has a position on the electromotive force series ofmetals between said metal of said base sheet and said metal of saidmetallic coating and which forms a water insoluble hydroxide, forming animage thereon of local areas masked with an acid-resistant coating,removing said surface metal coating from the unmasked areas of saidplate, contacting said partially masked plate with said acidic solutionand thereby create an electrical potential across the interfaces betweenthe masked and unmasked portions of said plate, continuing said contactof said acidic medium with said plate whereby a hydrosol comprising themetal of said dissolved salt is formed in said solution adjacent saidinterfaces which precipitates at said interfaces creating a bank toprotect relieved portions of the plate from undercutting simultaneouslywith their formation.

11. The method of etching a printing image in a bimetallic printingplate comprising a base sheet consisting principally of magnesium metaland a surface coating consisting principally of zinc metal having localareas thereon masked with an acid-resistant coating and simultaneouslybanking the sides of the relieved portions of the etched plate toprotect them against undercutting which comprises contacting saidpartially masked printing plate with an acidic aqueous medium which willcorrode the unmasked areas of the plate and thereby create an electricalpotential across the interfaces between the masked unetched zinc coatingareas of the plate and the unmasked etched magnesium metal areascontaining dissolved therein an aluminum salt, and continuing saidcontact of acidic medium with said plate whereby an aluminum hydroxidehydrosol is formed in the medium adjacent said interfaces and iselectrostatically attracted to said interfaces forming a precipitate atsaid interfaces creating a bank to protect relieved portions of theplate formed by the etching process from undercutting.

References Cited in the file of this patent UNITED STATES PATENTS2,026,603 Zarse Jan. 7, 1936 2,062,028 Howey Nov. 24, 1936

1. THE METHOD OF ETCHING THE SURFACE OF A METAL OBJECT HAVING LOCALAREAS THEREON MASKED WITH AND ACID-RESISTANT COATING AND SIMULTANEOUSLYBANKING THE SIDES OF THE RELIEVED PORTIONS OF THE ETCHED SURFACE TOPROTECT THEM AGAINST UNDERCUTTING WHICH COMPRISES CONTACTING SUCHPARTIALLY MASKED SURFACE WITH AN ACIDIC AQUEOUS MEDIUM WHICH WILLCORRODE THE UNMASKED AREAS OF SAID SURFACE AND THEREBY CREATE ANELECTRICAL POTENTIAL ACROSS THE INTERFACES BETWEEN THE MASKED ANDUNMASKED PORTIONS OF SAID SURFACE, PROVIDING IN SAID SOLUTION A HYDROSOLAND CONTINUING SAID CONTACT ACIDIC MEDIUM WITH SAID SURFACE IN THEPRESENCE OF SAID HYDROSOL TO FORM A PRECIPITATE AT SAID INTERFACESCREATING A BANK TO PROTECT SAID RELIVED PORTIONS FROM UNDERCUTTING.